In recent years, water-absorbing resin particles have been widely used in various fields of, for example, hygienic articles such as disposable diaper and sanitary articles; agricultural and horticultural materials such as water-retaining materials and soil conditioners; and industrial and construction materials such as water blocking materials for cables and dewfall preventing materials. As the water-absorbing resin particles used in such fields, there have been known, for example, a hydrolyzed starch-acrylonitrile graft copolymer, a neutralized starch-acrylic acid graft copolymer, a saponified vinyl acetate-acrylic ester copolymer, and a partially neutralized polyacrylate. Generally, it has been desirable for water-absorbing resin particles to have high water absorption, an excellent water-absorption rate, high swelling capacity, and an appropriate median particle size in accordance with the uses.
Among these, water blocking materials for cables are formed of two or more liquid-permeable sheets and water-absorbing resin particles that are fixed between the sheets, if necessary using an adhesive and the like. The demand for such water blocking materials has increased with development of electrical industry and communication industry. Water blocking materials for cables are used to wrap the cores of cables such as power cables and optical communication cables, and thereby the cores are protected. Then, the outside of the water blocking materials is covered with materials such as rubber. Thus, cables are formed. If the outer materials of cables such as power cables and optical communication cables are deteriorated to produce cracks and moisture enters through the cracks and reaches the cores of the cables, reduction in electric power and communication noise may be caused. The water blocking materials prevent such problems. The water blocking materials absorb such moisture and swell to increase the pressure in the cables, and thereby moisture is prevented from reaching the cores of the cables.
It has been also desirable for a water-absorbing resin used as a water blocking material that is used for cables (e.g., power cables and optical communication cables) to have high absorption capacity of liquid with a high salt concentration such as seawater. In order to achieve such absorption capacity, the following methods are suggested: a method of polymerizing an amino group-containing water-soluble ethylenically unsaturated monomer with acrylic acid in the presence of a crosslinking agent (see Patent Literature 1); a method of mixing a water-absorbing resin with an anionic surfactant (see Patent Literature 2); and a method of coating the surfaces of water-absorbing polymer particles with a water soluble resin solution (see Patent Literature 3).
However, specific materials need to be used in these methods, which leads to an increase in production costs. Rather, use of a large amount of a conventional water-absorbing resin often reduces the costs of a water blocking material and improves the performance of a water blocking material. Therefore, such conventional methods have brought not so great effects to the industries.
Further, a water-absorbing resin used as a water blocking material needs to prevent water penetration from the outside owing to cable damage early and maintain a water blocking effect for a long time. In addition to these, the water-absorbing resin needs to be efficiently formed into a water blocking material and have excellent handling performance as powder in the production of the water blocking material. Therefore, in order to achieve such performances, water-absorbing resin particles used for a water blocking material need to have high swelling capacity, a high water-absorption rate, and an appropriate particle size that achieves good handling performance.
One way of improving swelling capacity of water-absorbing resin particles is to control crosslink density thereof. For example, the following methods are suggested. A method in which reversed-phase suspension polymerization of an acrylic acid/acrylate aqueous solution is carried out in the coexistence of a surfactant with an HLB of 8 to 12, a crosslinking agent is added thereto (immediately after the polymerization), and a crosslinking reaction is carried out (see Patent Literature 4); and a method in which a percentage of water content of a carboxyl group-containing polymer is set at 10 to 30 wt %, and a crosslinking reaction of the surface is started (see Patent Literature 5). However, even these methods do not achieve high swelling capacity that is needed for water-absorbing resin particles used for a water blocking material.
Therefore, a technology for producing water-absorbing resin particles having high equilibrium swelling capacity, a high water-absorption rate or high initial swelling capacity, and an appropriate particle size that achieves good handling performance is desirable.